1881.] PROF. F. J. BELL ON THE ECHINOMETRID^. 416 



downwards to which the test is subjected, and to the widening-out 

 of the composite plates during the process of growth. 



I have dwelt at this length on the results of Prof. Loven's 

 labours, not merely for the purpose of directing again attention to 

 them ', but with the more especial aim of showing that it is only on 

 a misconception of the history that one can speak of secondary 

 plates as different from those first formed, or of such being added on 

 to the sides of a primary plate. But the origin of such a miscon- 

 ception is not far to seek ; it must surely be due to a study of the 

 arrangement of the pores of the adult, and be comparable to the 

 formulas of Milne-Edwards and Haime as applied to the structure 

 of the coral-septa ; while M. Loven's work will stand no less on an 

 equality with the elegant and instructive researches of Lacaze- 

 Duthiers". 



Armed with this knowledge we come now to a consideration of 

 the value of the characters of the arcs of pores. It has been pro- 

 posed to distinguish the family of the Echinometridse from the 

 Echinidse proper on the ground that the former have always more 

 than three pairs of pores to each arc, " while in the Echinidse the 

 arcs are always composed only of three pairs." " This division, 

 although it appears a numerical one, is yet one of great physiological 

 importance, as the mode of growth of the poriferous zone in these 

 two families is totally unlike " ^. 



I am inclined to think that the accomplished author is here using 

 the term physiological in some other sense than that to which its 

 etymology and the current usage of qualified persons justly entitles 

 it ; he is too experienced a zoologist to attempt to make the 

 functions of organs do the work of morphological and embryological 

 data. However, the mode of growth of the pores is as much matter 

 for morphologists as for physiologists ; and the only question which 

 really arises here is, as to the real character of this total unlikeness. 

 If such exists, it may or may not be of value. But, first of all, does 

 it exist ? 



Prof. Loven says'* : — " Les chiffres par lesquels la disposition des 

 pores est designee chez cette espece, les 2, 3, 3, 4, etc. de la serie 

 I a . . V i, et les 2, 2, 3, 4, etc. de la serie I 6 . . V a, se retrouvent 

 non seulement dans les especes voisines, le Toxopneustes drevispinosus 

 (Risso) et le T. lividus (Lamk.), mais encore dans le Loxechiims 

 albus (MoL), VEchinus esculentus, L., le Lytechims variegatus 

 (Lamk.), le Tripneustes ventricosus (Lamk.), ]a. Boletia heteropora, 

 Desor, V Amblypneustes ovum (Lamk.), le Temnopleurus toreumaticus 

 (Leske), VEchinothrix turcarum, Peters, V Echinocidaris imnctulata 

 (Lamk.), en un mot chez tous les Echinides. Les Echinometra n'y 

 font pas exception." 



So far, then, as the formation of the two separate families Echi- 

 nometridse and Echinidse is based on the difference in the mode of 



1 A short account is to be found in Prof. Huxley's ' Anatomy of Invertebrate J 

 Animals' (1877), p. 568. 



^ Arcbiv de Zool. Exp. vol. i. 



' Rev. of the Echini, p. 423. * T. c. p. -JG. 



